Image sensing elements used in an electronic still camera and the like are mainly interline CCDs (Charge-Coupled Devices). A conventional image sensing apparatus using the interline CCD is illustrated in FIGS. 7 and 8.
The conventional image sensing apparatus shown in FIGS. 7 and 8 includes an image sensing element 100 which is formed by an interline CCD, and an analog signal processing circuit 102 which is connected (AC-coupled) to the signal output side of the image sensing element 100 via a capacitor 101.
As shown in FIG. 7, the image sensing element 100 is comprised of a plurality of photodiodes 1 serving as photoelectric conversion elements which form a plurality of pixels two-dimensionally arrayed in the vertical direction (column direction) and horizontal direction (row direction) on the CCD image sensing plane for receiving incident light that forms an optical image of an object to be sensed, and photoelectrically convert light incident on pixels into signal charges corresponding to the intensity of light, read gates 2 which read out signal charges stored in the photodiodes 1, vertical transfer portions 3 which vertically transfer signal charges read out via the read gates 2, a horizontal transfer portion 4 which horizontally transfers signal charges vertically transferred from the vertical transfer portions 3, and a charge detection portion (output portion) 5 which converts signal charges transferred to the horizontal transfer terminal (output terminal) of the horizontal transfer portion 4 into a signal voltage and outputs the signal voltage.
The charge detection portion 5 is formed by, e.g., a known floating diffusion amplifier (FDA) connected to the transfer terminal of the horizontal transfer portion 4. The floating diffusion amplifier comprises a capacitor 51 which forms a floating capacitor for storing signal charges transferred to the transfer terminal of the horizontal transfer portion 4, a reset MOS transistor 52 having a reset pulse φR application gate electrode for resetting a potential determined by signal charges stored in the capacitor 51 to a predetermined reference potential for each pixel, a reference power supply 53 for a reference potential that is connected to the drain of the MOS transistor 52, and an output amplifier (buffer) 54 which is formed by a plurality of source follower circuits using, e.g., MOS transistors, converts signal charges stored in the capacitor 51 into a signal voltage corresponding to a change in potential with respect to the reference potential, and outputs the signal voltage.
In the image sensing element 100 shown in FIG. 7, a region except the photodiodes 1 is covered with a light-shielding film such as an aluminum film, and shielded from light. Also in a region including the photodiodes 1, a horizontal optical black region 6 for obtaining a reference signal for determining the optical black (OB) level of an image signal is formed in the end region of rows in an effective pixel region 1a for obtaining an object image. In the horizontal OB region 6, the entire element surface including the photodiodes 1 is shielded from light. A signal detected by the charge detection portion 5 contains a signal photoelectrically converted by the photodiode 1 in the effective pixel region 1a, and a dark current component generated in the photodiode 1 and vertical transfer portion 3 in the horizontal OB region 6.
In the image sensing element 100, signal charges stored in the photodiodes 1 are read out, in accordance with a signal read pulse superposed on a vertical transfer pulse φV1, via the read gates 2 below the gate electrodes of the vertical transfer portions 3 to which the vertical transfer pulse φV1 is applied. Signal charges read out to the vertical transfer portions 3 are transferred row by row to the horizontal transfer portion 4 every horizontal scanning in accordance with vertical transfer pulses φV1 to φV4 of four phases. Signal charges of respective rows which are transferred to the horizontal transfer portion 4 are sequentially transferred to the charge detection portion 5 in accordance with horizontal transfer pulses φH1 and φH2 of two phases. Signal charges are converted into a signal voltage by the charge detection portion 5, and the signal voltage is applied as a CCD output signal to the analog signal processing circuit 102 via the capacitor 101 shown in FIG. 8.
The analog signal processing circuit 102 comprises a clamping circuit 103 which performs DC recovery operation, i.e., OB clamping of recovering the DC component of a signal AC-coupled via the capacitor 101 on the basis of an output signal (OB signal) from the horizontal optical black region 6 of the image sensing element 100, and a various-processing unit 104 which performs various signal processes such as gamma correction for an image signal generated by OB clamping. The clamping circuit 103 includes a reference power supply 106 for a reference voltage that is connected to the capacitor 101 via a switch 105.
While the image sensing element 100 outputs an OB signal from the horizontal optical black region 6, the switch 105 is turned on by a clamping pulse φOB to connect the reference power supply 106 to the capacitor 101, thereby setting the signal processing circuit side of the capacitor 101 to the same voltage as the reference voltage. While the image sensing element 100 outputs a photographing signal via the photodiode 1 in the effective pixel region 1a, the switch 105 is turned off by the clamping pulse φOB to generate an image signal for setting the OB signal to a DC-level reference (optical black level). OB clamping is executed for each row. After that, the various-processing unit 104 performs various processes to obtain a final image signal.
In the conventional image sensing apparatus described above, when the horizontal optical black region 6 of the image sensing element 100 suffers a light shielding error and light shielding is insufficient, or when a defect such as scratch exists in the photodiode 1 present in the horizontal optical black region 6 and a large dark current is generated, a redundant noise component is added to a signal from the horizontal optical black region 6, failing to obtain a reference signal of an accurate optical black level. The clamping circuit 103 executes erroneous DC recovery operation, resulting in image degradation such as a horizontal streak.